Fluid pressure control devices, such as control valves and regulators, are commonly used to control the flow characteristics of a fluid. A typical device includes a valve body defining an inlet, an outlet, and a fluid flow path extending between the inlet and the outlet. A valve seat is coupled to the valve body and defines an orifice through which the flow path travels. A throttling element, such as a plug, is moveable relative to the valve seat thereby to control fluid flow through the orifice. In a sliding-stem fluid control device, the throttling element is coupled to a stem extending outside the valve body, which in turn is coupled to an actuator for positioning the throttling element relative to the valve seat.
Sliding stem fluid control devices often require components for guiding the throttling element assembly with respect to the valve seat. In particular, it is desirable to guide the linear movement of the throttling element assembly so that it is concentric with the bonnet, packing bore, cage, seat ring, or other component coupled to the valve body. Close guiding of the stem and/or plug tip also maintains maximum lateral stability to resist vibration and fatigue failures. Accordingly, components which guide movement of the throttling element often include guide surfaces that slide against one another.
Rubbing and sliding of guide components in fluid control devices may cause material from the valve components to become free due to wear, galling, or other causes. The non-corrosive materials used for some applications are particularly susceptible to galling. Galling and other wear phenomena can cause movement and transfer of component material along the contact path. The loose material may degrade or disrupt sealed engagements within the fluid control device, such as the primary seal between a throttling element and seat, a secondary seal between a throttling element and cage, or a stem packing seal between a stem and packing assembly, to name a few.
Conventional approaches to reduce galling typically employ the use of dissimilar materials for the components which contact one another. This practice can result in higher cost materials and assembly, and may limit use of the device in certain applications.